


The Definition of an Egghead

by rehliamonster



Category: Underfell - Fandom, Undertale (Video Game)
Genre: Alien Biology, Alternate Universe - Underfell, Awkward Sexual Situations, Cranial Pregnancy, Eggs, Horror Comedy, I'm gonna be real here I have no idea how to tag this, M/M, Oviposition, Prostitution, Skull Fucking, Weird Biology, Weird Sex, dubcon, i think, sexual favours, technically body horror, weird pregnancy
Language: English
Status: Completed
Published: 2018-04-01
Updated: 2018-04-01
Packaged: 2019-04-17 01:10:29
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,397
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/14177322
Author URL: https://archiveofourown.org/users/rehliamonster/pseuds/rehliamonster
Summary: Sans often fucks up, but destroying the majority of Grillby's expensive alcohol is a new height even for him. Thankfully, Grillby already has an idea how he can take care of that debt... and it's a bit more unusual than Sans accounted for!





	The Definition of an Egghead

**Author's Note:**

> So, I wasn't done with the weird head pregnancies yet. Sorry. I swear I'll go back to more normal oviposition tomorrow.
> 
> ...WOW you know you're deep in weird kink land when oviposition ranks as 'more normal!' Off I go to kinkshame myself lmao

He knew he had fucked up.

That was a new feeling for Sans, because he fucked up quite often, actually. The fine difference was though, he usually fucked up in small and controllable ways, the kind of fuck up that ended up easily mended and he'd walk out with a funny story of wacky hijinks, something to tell to the bar patrons over a bottle of mustard while laughing uproariously. 

This wasn't small or controllable, nor was it funny. 

It had been an accident, true, but it had been an accident caused by him while he had been trying to tell one of the aforementioned uproariously funny stories to the bar patrons, and okay maybe he had been a bit plastered, or a lot, but the end of it was that he had fucked up big time and it was his fault and he was in _really big trouble_.

“So,” Grillby rasped, looking pointedly from the door through which he had just thrown out the last bar patrons to Sans and then down to the heap of glass shards at Sans’ feet. “Let's talk.”

Sans gulped. 

“uh, yeah, sure! hey, you know this wasn't on purpose, right?” Sans hurried to clarify. He wished he didn't find that so difficult now even though the alcohol he had ingested with his mustard had fled from the shock, leaving his mind painfully clear and sharp. Perhaps it was this shock that made it hard to focus. “i mean, shit, the bone just kinda flew outta my hand, yanno how it is - “

“I do not,” Grillby stated, entirely calm and mild, but he may just as well have punched Sans in the face. Grillby crossed his arms. Slowly shifted his weight from one foot to the other. 

Sans felt the sweat gather on his skull. 

“yeah. okay. i actually, uh, i believe that. look. grillbz. pal. we've known each other for, what. how long? gotta be almost ten years now. heh. nice round decade. anyway.” He was babbling. He couldn't afford to babble, he had to get back on track. He took great care to let his voice convey how serious he felt. “you know i’m good for it. right? i know i’ve dragged my lazy ass on payin’ my tab a couple ‘a times, but i always paid ya back. i’ll pay ya back now. i swear.”

“I know you will,” Grillby said, and managed to make it sound threatening in spite of his entirely relaxed demeanour. 

Damn. Could this fucker perhaps not pull this quiet and mysterious bullshit now? Sans felt he deserved better than that in spite of the mess he had created. 

“right. so, uh…” Sans eyed the floor, carefully taking in the frankly scary amount of broken glasses and bottles. It looked like almost the entire contents of the bar. It… might be. “how much exactly do i owe you?”

Grillby said a number. A number that was tremendously, stupidly high, a number whose weight was made worse by the fact that Sans knew it must be accurate. Many things could be said about Grillby, a lot of them unflattering, but the two most important were these: he always had his prices right and he never cheated his customers. He was fair, in his own way, which counted for a lot in a town where murder could be an acceptable solution to having someone look at you wrong. 

“fuck,” Sans spat. Then he immediately tried to amend this statement, not wanting to come across as if he wouldn't make good on his words. “i mean, uh. i might need some time. take an extra job. i could clean for you? wanna fuck again?”

Grillby seemed to consider the offer, which relieved Sans. It perhaps wasn't dignified, but spreading his legs was a low effort way of taking care of his debts, one that he'd used before with Grillby, who could be intense but was never too rough with Sans. He'd never been worried about his single hp while he was fucking Grillby, which also counted for a lot in Sans’ book. Plus, Grillby was attractive enough that Sans sometimes tried to hit on him even without any debts in the way. Sometimes the bartender even accepted. It had been a while since that last happened. 

Please say yes, Sans thought to himself. 

“I may have a proposal,” Grillby finally said, with a shrew undertone that immediately had Sans on guard. 

“okay, shoot,” he ventured. He didn't like that undertone, but he'd hear Grillby out. Not that he had much choice. Why couldn't they just have sex?

“Muffet's influence has grown recently,” Grillby began. 

Sans nodded; apart from Grillby complaining here and there about the war between his and her gang, his brother had told him about that too, had been excited because with all the trouble Undyne had with Muffet's underlings, the guard captain might just be willing to finally consider Papyrus for the guard just to have more manpower. Sans hoped that would happen. His brother had trained hard and watched the dogs during their rounds to study up on their moves, and hoped to one day become the leader of the pack instead of Dogaressa. If Papyrus got that position, Sans would have an easier time too thanks to the prestige and power of his brother. 

“She’s disrupting my supply lines,” Grillby continued. “Her clan is so large that she has an easy time staging ambushes on my henchmen. I pay them well, but Muffet has loyalty on her side. The spiders are her children. This is what I need.”

“uh,” Sans said intelligently, blinking as the implication made itself clear. “uh. you know i’m a skeleton, right? i can't get pregnant.”

“Perhaps not in the usual way, but us fire elementals don't reproduce in the usual way either,” Grillby explained. “All we need is an enclosed cavity inside another monster where our eggs have access to magic. The eggs are small, like sparks. They grow into flames over time until they're ready to hatch and form a corporeal body.”

“buddy, the only enclosed cavity i have is my skull,” Sans laughed, amused that Grillby wasn't getting it. 

A moment of silence between them, as it occurred to Sans that he had been the one not getting it. 

“you want to lay eggs in my skull?!” Sans tried to clarify. 

“It would help me a great deal,” Grillby said pleasantly, as if they were discussing Sans doing the evening chores instead of whatever this madness was. “Of course I have considered your fragile constitution. There is no danger to you, believe me. You will merely feel tired for the duration as my spawn feeds on your magic, and I would of course protect you to ensure that your exhaustion doesn't tempt potential attackers. You would spend the month of incubation in leisure and comfort.”

Well, wasn't this shit just wild. 

It almost felt as if their roles were suddenly reversed, as if Grillby was asking Sans for a huge favour and tried to make the pitch sound as appealing as possible. Parts of it did sound appealing. Sans was always up for a month of doing nothing. But on the other hand…

“man, i… i dunno grillby. inside my skull?” he mumbled, reaching up to cradle the smooth plane of bone in his hands. He could put things inside there, that was true. He and his brother used to annoy each other by dropping small items into the other's sockets when they were small. It was uncomfortable, but it didn't hurt and the items came out easily enough again. It was just... his skull was sturdy, but it wasn't indestructible. As the gold tooth and the deep crack spidering away from it showed, it could be damaged. The injury had hurt like hell for several months until it finally started to scar over. 

And a clutch of eggs was different from a ping pong ball or whatever else he and Paps had thrown into their skulls. 

“I would clear all your debts, from this evening and whatever you currently have on you tab,” Grillby argued. “All in one, no further payments or favours needed.”

It was tempting. Very tempting. The number Grillby had told him earlier still bounced around in Sans’ mind, and he knew that it would usually take him several months of hard work just to clear today's worth, his tab not included. 

“I could talk to the dogs too,” Grillby negotiated. “Mention how the skeleton brothers are doing their part to keep Snowdin stable and safe. Slide in a few coins to make sure those words reach the captain.”

Sans knew he had to say yes. He knew. Papyrus might insist that he wanted to get into the guard on his own efforts and work up the ranks, but everyone knew that as much as Undyne had improved the guard and held them to a high standard, any amount of progress was the result of greasing the right hands. Money and favours ruled the guard just like they ruled the rest of the underground. That, and murderous intent. Papyrus secretly lacked much of the latter so he'd need more of the former, and this was an offer on a silver platter that wouldn't come around again. Grillby’s word had weight, even with the guard, even though he was a gang leader. In comparison, Sans and his brother were nobodies.

They could be more.

“will it hurt?” he dared to ask, immediately feeling childish for the question. 

“I don't think it will,” Grillby said, deliberately soothing. “None of the other monsters carrying fire elemental eggs ever complained. They only said is was very warm. You might sweat a lot.”

“i already sweat a lot,” Sans grumbled. 

Grillby nodded his head, as if that was an additional argument in favour.

“how will those eggs even get in there?” Sans questioned. 

A grin broke out on Grillby’s face, one that Sans knew very well. It was the same grin Grillby always wore just before they fucked. 

“I am reasonably certain my cock will fit into your eye socket,” Grillby said. 

The absurdity of the whole situation hit Sans all at once and he couldn't help himself, he began to laugh out loud. 

“you wanna dick my eye socket. seriously? shit man, and here i thought i was a nasty kinkster. sure, whatever, knock yourself out,” he cackled. 

“So you agree?”

“probably. yeah. yeah i do. ‘s a good offer,” Sans sighed, secretly wondering if he was going to regret this. 

“Very good. Then I think you should probably come over and kneel down,” Grillby said, cocking a finger at Sans to indicate he should approach. 

“what, we're doing this now?” Sans asked. 

“Why waste time? The faster this will be over, the happier we will both be,” Grillby reasoned. “I'll have my loyal children to expand my clan, and you'll be free of your debts and on the way to a better social standing thanks to your brother's promotion.”

“oh man,” Sans gulped. “fine. ‘kay.”

It still took him some willpower to walk over to Grillby and lower himself down onto his knees. He tried to concentrate on the upsides instead of the weirdness of it all, the month of lazing around and the easy rise up in the pecking order after it was over. It would be worth it. 

Grillby opened his pants and stroked himself for a bit, preparing himself for the act. Sans stared ahead at the steady movements in disbelief. A cock in his eye socket. In terms of weird shit he'd done in his life, this definitely had to rank in the first spot. 

Grillby gently placed his hands on the sides of Sans’ skull, lining himself up. 

“Open up wide,” he instructed. 

Sans did, a nervous giggle escaping him. 

That quickly became an indignant squawk as Grillby pushed inside his left socket, going slowly. Sans’ vision in that eye sputtered out, leaving him with a lopsided field of view. 

“Are you okay?” he asked, making Sans feel like some dumb virgin in a bullshit Mettaton production. Just with less blood and post-coital maiming. 

“yup. kinda weird, not gonna lie, but it don't hurt,” Sans said truthfully. He felt dumb kneeling on the floor with his sockets wide open while Grillby began to pump in and out of one. Was moaning too, the freak. Sans couldn't believe Grillby was apparently getting off to this. If they ever had sex in the usual way again, Sans would tease him mercilessly for being so into this. 

Personally, he didn't get the appeal. It was merely a strange, slightly tingly sensation in his socket. The magic in his skull was mildly agitated by Grillby’s movements, but otherwise he felt nothing. He tapped his phalanges against his shorts, wondering idly how he would explain this to his brother. He'd have to have some sort of excuse that didn't betray the deal for the guard position… 

Perhaps he would just leave that part out and be honest about the rest for once. Papyrus would probably be weirded out and then mock him for having to do something like this to settle his debts, followed by endless nagging about how Papyrus kept saying that Sans should stop it with the tab. Was it weird that he was thinking about his brother during this? Probably. It just didn't feel sexual to him though, that was the thing. Still, he could think about something else. Quantum equations. 

That helped him pass the time until he heard Grillby moan loudly, his movements sputtering. Sans readied himself just in time, and then he felt the surreal sensation of cum and something else hit the back of his cranial cavity. 

Just like Grillby said, it was very warm, almost hot but staying just short of uncomfortable. He could feel the flickering sparks, little burning globes that clustered together on the inner walls of his cranium. They were tiny, but numerous, too numerous to try and count them as they stacked over each other. Grillby’s cum wrapped around them, seemed to cushion them. It felt sticky, but not painful either. It took a while before Grillby was apparently done and pulled out again. 

“How are you feeling?” he asked immediately, looking at Sans with a serious expression. 

Sans dared to blink, moving his head back and forth to gauge how the eggs would move in his skull. There was a definite change in weight, his skull noticeably heavier than before. He could feel them slide around a bit, but not too much. Grillby’s sticky cum kept them in place. Only the fact that they were still sparking and giving off heat was mildly annoying, because it kept causing his skull to itch and made him feel very warm. He could see how sweating would become a problem in the coming month. Other than that though, not the worst feeling he'd ever experienced. 

“it’s fine,” he said. “kinda itchy and warm. but i can take it.” 

“I'm glad to hear it” Grillby said warmly. Heh. “Sit down while I clean this up. I'll bring you home to make sure you get there safely, and help explain things to your brother. Without mentioning our little extra arrangement, of course.”

Wow. Sans felt his brow bones rise up as he settled into a booth, getting comfortable while he watched Grillby zip his pants up again and start cleaning his bar. This was real nice, to be honest. All he had done was hold his socket open for a bit while Grillby did his thing, and now the bartender was treating him more nicely than anything Sans was used to. He hadn't even needed to pretend to be into the dicking. And now he got to watch while Grillby cleaned up the mess Sans had made earlier. 

Apparently Grillby was very intent to keep up his part of the bargain, and Sans really wouldn't have to do anything at all. 

Totally worth it.

*

“you absolute _fucker_ ,” Sans roared, not caring about his language or who heard him. “this wasn't worth it! i’ll kill you! i’ll fucking dunk you into the waterfalls you useless pile of coals!”

“Shhh, you're doing very well,” Grillby told him, gently rubbing his back. 

“get offa me! get this fucker away from me!” Sans yelled. 

“PERHAPS YOU SHOULD REMOVE YOUR HANDS AND LET ME DO THE BACK PETTING,” Papyrus snarled, who for some reason had insisted to be there during the ‘birth.’ Something about not fully trusting Grillby with his brother’s fragile health. 

“Of course. I only wanted to be supportive,” Grillby said. 

“fuck you and your support!”

“HOW MUCH LONGER IS THIS GOING TO TAKE?”

“Not much, I think, the fire is already leaking out of his sockets.”

Sans growled, wanting to bite Grillby’s hands off. Or his cock perhaps. 

There was indeed fire leaking out of his sockets, burning high and tinting his brow bones black, while a liquid substance not unlike lava leaked out at the bottom like some sort of fiery tears. Papyrus had specifically put some towels underneath him for the moment, preparing their living room where he had insisted the birth should take place, although Sans suspected they wouldn't be enough if this continued the way it was going now. Smoke billowed over his head, steaming away from the fire that kept dripping out of his eyes. 

He had a splitting headache from whatever the fuck was going on inside of him. 

It wasn't actually hurting him per se, his hp still sat steadily at one, but it was still incredibly painful. It stung, it burned, it felt as if his skull would simply crack open at any second now. He had mostly gotten used to the sweltering heat over the course of the past month, had easily dealt with the sweat and the increasingly itchy sensation of the fire that kept building on the eggs Grillby had laid in his skull. Even the increasing heaviness as the eggs grew had been manageable as he was pampered by Grillby, who had gone so far as to feed him when he hadn't felt like getting up. Paps had been appalled at the amount of laziness Sans had been able to get away with just because of his strange sort of pregnancy, although he had also kept laughing at Sans when the eggs in his skull had caused his eye lights to be permanently on fire, which Sans had to admit looked cool at first glance and really dumb from the second onwards. If even Papyrus thought it was ridiculous, that's when you knew it was too cliché. Still, he had dealt with it. 

But this was too much. 

No month of being utterly pampered could make up for this. 

“Ah yes. See? This is a sign that it's beginning,” Grillby said, apparently pointing at something. 

“WELL, ISN'T THAT GOOD NEWS, BROTHER. SOON YOUR SKULL WILL BE JUST AS EMPTY AS IT ALWAYS WAS AGAIN!” Papyrus teased him. “THEN YOU CAN GO BACK TO BEING JUST A REGULAR OLD EGGHEAD! NYEH HEH HEH!”

“fuck you,” he groaned. He couldn't even appreciate the fact that his brother made a pun. His head hurt too much and his brother's loud voice didn't exactly help matters. 

A loud crackle resonated from inside his cranium and Sans whined as the heat increased, with more smoke pouring out of his sockets. He couldn't see a thing and the smoke stung when he inhaled it, making him cough. 

“There goes the first one,” Grillby stated happily. 

Sans choked on a reply he couldn't manage to get out. Something was suddenly wiggling inside his skull, crawling around in it. 

“hiiiiiiiiiiieeeeeeeh!” 

Had that been him? No. He refused to believe that such a high pitched shriek could have come out of his own throat. 

“Come to daddy, little one,” Grillby cooed. 

“don’t say that,” Sans complained. He really didn't want to hear that kind of shit coming out of the bartender’s mouth right now. He’d never be able to enjoy it again if Grillby said it when they fucked. 

The crawling thing in his skull reached his socket and skittered past the opening and presumably into Grillby’s hand. 

“Beautiful,” Grillby stated with a sound of deepest satisfaction. “What a big and healthy spawn. They're coming out very well, Sans.”

“i want to die,” Sans whined, already feeling another fire elemental burst out of its egg inside of his cranium and beginning to skitter out. 

“OH BROTHER, DON'T BE OVERDRAMATIC!” Papyrus scolded him. “PERHAPS THIS UNUSUAL CRANIAL PREGNANCY WILL REMIND YOU TO BE MORE CAREFUL WITH YOUR DEBTS FROM NOW ON!”

“do you have to nag me now?! while my skull is filled with flaming babies?” Sans complained. 

“WELL, WHAT ELSE DO YOU WANT ME TO SAY, SINCE MY GENUINE SUPPORT UPSET YOU?” Papyrus huffed.

“you could say literally anything else!” Sans said, squeaking when the next fire baby tumbled out of his socket. 

“LIKE STATING THE FACT THAT THERE'S AN “EGG” IN THE WORD PREGNANCY?” 

“i fucking know that! you stole that line from me!” 

“I THOUGHT YOU MIGHT LIKE HEARING YOUR OWN EGGS-CELLENCE REPEATED BACK AT YOU.”

Sans choked on more smoke that billowed out of every opening of his skull. 

“I'M REALLY WILLING TO PUT IN EGGS-STREME EFFORT TO CRACK THEM AT YOU.”

“stop,” Sans begged. 

“What an amusing reversal of your usual roles,” Grillby noted. 

Sans wanted to tell him what he thought of Grillby’s amusement, but he only ended up sputtering when he felt several of the fiery eggs in his skull burst open at once and the whole cavity was suddenly filled with rapidly moving fire creatures. They barely moved out before the next one followed, and it felt as though fiery columns were shooting out of Sans’ eyes. He felt like throwing up. 

When he opened his mouth, more fire elementals poured out, scalding his throat. He felt them hit his legs and the towels underneath him, burning straight through. 

They kept coming and coming while he was unable to do anything about it, he couldn't even scream. 

“NOT ON THE FLOOR! MY CARPET!” Papyrus screeched, sounding appalled. 

“I can't hold them all!” Grillby defended himself, “The bucket is overflowing!”

“YOU SHOULD HAVE BROUGHT A BIGGER BUCKET! OR A CRADLE! WHO KEEPS THEIR BABIES IN A BUCKET ANYWAY!” Papyrus complained. 

“They're only blobs of fire with hands right now,” Grillby reasoned. “They'll need another week to grow proper limbs.”

“HOW IS THAT AN EXCUSE?” 

“can you two just fucking shut up?!” Sans managed to press out when the flood finally stopped. It didn't bring him any relief, only another problem. “i think one is stuck.”

“Let me see,” Grillby said at once, and gently touched the sides of Sans’ skull to tilt it back. 

The fire elemental in his skull didn't move with it, merely squirming in place. Sans felt Grillby poke his fingers into his eye socket and snarled, barely stopping himself from snapping at the other. It was already full enough in there, he didn't want more foreign objects in his skull, but he knew it was the only way to get the fucking thing out of him. 

“Steady,” Grillby told him. “Keep it wide open.”

Sans grunted, but followed along, keeping his sockets as wide as he could. Grillby was twisting his fingers inside Sans’ skull, trying to reach the elemental. It took him a moment until he managed to grab it and began to pull. It slid slowly out of his skull, and Sans felt it press against the edges of his socket as it was pulled out and couldn't believe something so massive was sitting inside his cranium right now. He should probably be glad that the thing wasn't so big that it didn't fit through his socket at all, but all he felt was nausea as the elemental was slowly pulled out of him. 

The edges of his eye socket stung when Grillby finally managed to pull the thing free with a sickening pop. 

“There we go,” Grillby said happily. “What a nice, big flame you gave me.”

“i’m gonna give a nice big fist to go with it if ya don't shut up,” Sans snapped, panting hard. 

He was covered in sweat, but with this elemental, the last of it seemed to be over. There were merely remnants of semi-liquid fire dripping out of his skull. Ever so slowly, his vision flickered back in fully, allowing him to see the carpet. Or what was left of it, burnt and blackened and half-molten thanks to the partially artificial fibres. Nothing remained of the towels and his shorts were burnt halfway up his femurs too, leaving the bone blackened. His skull probably didn't look any better. 

When he looked up, he saw Grillby kneeling in front of him, flickering happily next to a bucket full of crackling baby elementals. The final one that had been pulled out of him was laying in Grillby’s arms, and was the only elemental that already had a more developed body. Underneath the deep red fire, a tiny skeleton was visible. 

“IT SEEMS THAT ONE TAKES AFTER YOU,” Papyrus points out, standing behind Grillby with crossed arms. His expression wasn't as amused as Sans expected, instead it was a scowl laced with barely concealed worry. 

“heh. must be,” Sans gasped. “stubborn fat asshole.”

“INDEED,” Papyrus agreed with him. Ruthless as always, but Sans knew he was only masking the fact that the last baby being stuck had scared him. “SO? DO YOU HAVE ANYTHING ELSE TO SAY?” 

Their gazes met across Grillby’s head. They stared at each other for a long moment while Grillby looked between them with mild confusion. An understanding passed between Sans and his brother and Sans looked down again to meet Grillby’s eyes. 

“grillbz.”

“Yes?”

Sans took one last quick glance at the baby fire elementals before he squared up and looked back at Grillby with something almost like determination. 

“i’m never gonna drink alcohol again,” he declared solemnly. 

No more head babies from him. Once was more than enough.

Grillby’s expression flickered between surprise and something approaching regret when he followed Sans’ gaze and saw how serious he was. 

“Aw. That’s a shame,” he had the gall to say. “Are you sure?”

“get out!”


End file.
